1. Field of the Invention
The present invention relates to an X-ray tube having a characteristic electron gun.
2. Description of the Related Art
An X-ray tube includes an electron gun with a typical structure in which a Wehnelt electrode is formed with an opening inside which a coil filament is disposed. The coil filament emits an electron beam which is narrowed by an electric field made by the Wehnelt electrode to make a specified electron-beam-irradiated region on a target, so that the irradiated region generates X-rays. The electron-beam-irradiation region emits not only X-rays but also metal atom ions, i.e., positive ions, the metal atom making up the target material. The ions may occasionally collide with the filament. When the filament experiences the ion bombardment, the filament is subject to erosion disadvantageously, resulting in a shorter lifetime of the filament.
Then, there have been known the countermeasures in which the position of the filament is shifted from the position facing the electron-beam-irradiation region so that the filament experiences the ion bombardment as little as possible. FIG. 1 shows in section such an eccentric filament configuration. FIG. 1 shows a condition in which an electron gun 12 faces a revolving target 10 (i.e., rotating anode). The electron gun 12 includes a Wehnelt electrode 14 and a coil filament 16 which is disposed inside an opening 18 formed in the Wehnelt electrode 14. The opening 18 and the filament 16 extends long in a direction perpendicular to the drawing sheet. A line 22, which passes through the center-of-width of the filament 16 and yet is perpendicular to the front face 20 of the Wehnelt electrode 14, is referred to as a filament center extension line hereinafter. The eccentric filament configuration has a feature that the center-of-width of the electron-beam-irradiation region on the target 10 is deviated from the filament center extension line 22 by a distance D which is about a half width of the filament 16. In other words, the opening 18 of the Wehnelt electrode 14 is formed asymmetric about the center-of-width of the filament 16 so that the electron beam 24 can be deviated as described above. A distance A between the filament center extension line 22 and one longer side 26 (which extends in a direction perpendicular to the drawing sheet) of the opening 18 is different from another distance B between the filament center extension line 22 and the other longer side 28 (which also extends in a direction perpendicular to the drawing sheet) of the opening 18, the distance A being shorter than the distance B. Accordingly, the electric field made by the Wehnelt electrode 14 affects the electron beam 24 asymmetrically, so that the electron beam 24 is deflected downward as shown in FIG. 1, resulting in the deviation of the electron-beam-irradiation region by the distance D as described above.
FIG. 3 shows a positional relationship between the opening 18 of the Wehnelt electrode and the filament 16. The opening 18 and the filament 16 each has an elongate shape as a whole. The distance A between the center-of-width line 34 of the filament 16 and one longer side 26 of the opening 18 is different from the distance B between the center-of-width line 34 and the other longer side 28, the longer sides 26 and 28 being straight lines.
In the field of the X-ray tube, the electron gun with the eccentric filament configuration is known and disclosed in, for example, Japanese patent publication No. 5-242842 A (1993), which is referred to as the first publication, and Japanese patent publication No. 2001-297725 A, which is referred to as the second publication.
The first and second publications each relates to a structure having a combination of a couple of the eccentric filaments and discloses the formation of an opening asymmetric about the filament so as to deviate the electron-beam-irradiation region on the target from the above-mentioned filament center extension line.
The inventors of the present invention have found out that the electron gun with the eccentric filament configuration gives rise to a curved electron-beam-irradiation region on the target. FIG. 8A shows the shape of the curved electron-beam-irradiation region as viewed from the left side of FIG. 1. The electron beam is narrowed and deviated downward as shown in FIG. 1 so that the electron-beam-irradiation region 30 is curved with a downward convex shape as shown in FIG. 8A. Denoting the length of the elongate electron-beam-irradiation region 30 by a symbol L and the width by a symbol W, the length L is 8 millimeters and the width W is 0.4 millimeter for example. Assuming that one end of the width (the upper end in FIG. 8A) at one end of the length of the elongate electron-beam-irradiation region 30 is connected by a line segment 32 with similar one end of the width at the other end of the length, the maximum distance between the line segment 32 and the curved longer side is defined as a curvature amount which is denoted by a symbol ΔW. The curvature amount ΔW is divided by the width W of the electron-beam-irradiation region 30 to get a value ΔW/W which is defined as a curvature coefficient. In the case of the conventional shape of the opening 18 shown in FIG. 3, the electron-beam-irradiation region 30 is curved as described above and its curvature coefficient is about 0.02 for example.
The curved electron-beam-irradiation region would give rise to some problems described below. FIG. 10A is a perspective view showing an X-ray optical system in which a line focus 52 emits an X-ray beam 54 which is reflected by an X-ray reflecting mirror 56 made of a synthetic multilayer film and then irradiates a sample 58. It should be noted that although the optical system uses an elliptic mirror to get a focused X-ray beam, a parabolic mirror may be used to get a parallel X-ray beam. When the X-ray optical system uses such an X-ray reflecting mirror, it is very important to keep the linearity in shape of the line focus 52. A positional relationship between the X-ray reflecting mirror 56 and the line focus 52 must be exactly determined so that an X-ray beam 60 with a sufficient intensity can be taken out from the X-ray reflecting mirror 56. It is important in this case to exactly keep the predetermined positional relationship between the X-ray reflecting mirror 56 and the line focus 52 at any longitudinal position on the line focus 52. If the linearity in shape of the line focus 52 is kept with a high degree of accuracy, the positional relationship with the X-ray reflecting mirror 56 is constant at any longitudinal position on the line focus 52, so that an X-ray beam 60 with a sufficient intensity can be taken out.
On the contrary, if the line focus 53 is curved as shown in FIG. 10B, even though the longitudinal midpoint of the line focus 53 has been exactly positioned with the predetermined positional relationship with the X-ray reflecting mirror 56, the longitudinal ends of the line focus 53 would be misaligned with the predetermined positional relationship. As a result, even though an X-ray beam 62 with a sufficient intensity is obtained from the vicinity of the longitudinal midpoint of the line focus 53 through the X-ray reflecting mirror 54, an X-ray beam 64 from the vicinities the longitudinal ends of the line focus 53 through the X-ray reflecting mirror 54 would have a lower intensity than from the vicinity of the longitudinal midpoint. Since the degree of accuracy in positioning between the X-ray reflecting mirror and the X-ray source strongly affects an intensity of an X-ray which is taken out from the X-ray reflecting mirror, the linearity in shape of the line focus (i.e., the degree of curvature) would strongly affect an X-ray intensity from the X-ray reflecting mirror.
Since the shape of the line focus corresponds to the shape of the electron-beam-irradiation region on the target, it is very important to make the curvature coefficient of the elongate electron-beam-irradiation region as small as possible in the X-ray optical system using the X-ray reflecting mirror. If an X-ray beam from the X-ray source irradiates a sample directly, the curvature coefficient of about 0.1 for example would almost always have no problem. On the other hand, if using the X-ray reflecting mirror, the curvature coefficient of about 0.1 would have a problem that an X-ray intensity from the X-ray reflecting mirror would be reduced by about ten percent as compared with the case using the linear line focus. Accordingly, when using the X-ray reflecting mirror, the curvature coefficient of the electron-beam-irradiation region should be as small as possible, not greater than 0.01 being preferable. However, when adopting the above-described eccentric filament configuration, the curvature coefficient of the electron-beam-irradiation region would become about 0.02 disadvantageously with no countermeasure in the shape of the opening of the Wehnelt electrode, so that there is a problem with a reduced X-ray intensity which is taken out from the X-ray reflecting mirror